At the start of the whisky-making process, the barley germinates and its starch content turns into sugar. Then, the producers have to dry the malt. If done over a hotbed of peat, the finished product takes on smoky notes.
Next, grinding or milling the dried barley turns it into a coarse flour called grist. The grist must be of the right grade; if too rough, it will not be possible to extract all the sugar. Conversely, if it is too fine, it clumps together – also impeding full extraction.
Following centuries-old traditions, the distillers then pass piping hot natural spring water through the grist. In doing so, the malt sugar dissolves and yields a naturally sweet liquid extract known as worth. Depending on weather conditions, the distinct aroma of mashed cereal is detectable for miles around the distillery.
A sequence of three mashings uses progressively hotter water each time – at about 65, 80 and 95 degrees Celsius – to extract the maximum. Between cycles, the worth passes through a cooler. During the final iteration, the extra sugar yield is relatively low, so the proceeds go to the next batch.
Adding yeast to the worth
In the cooler, after mashing, the sugar solution must cool down to 20°C. Then, in a large vessel called a washback, the addition of yeast causes the worth to ferment. To give an idea of the scale, some 50 kilogrammes of yeast are necessary for 15,000 litres of worth.
During the fermentation, lids on the washbacks reduce the entry of bacteria to minimise the risk of the alcohol turning into vinegar. Characteristically, the vessels have a horizontal blade on top, which rotates and cuts the foam.
Importantly, wash boxes are resistant to fungi. While Oregon pine or cypress wood materials are traditional, they require regular cleaning and maintenance. For this reason, some distilleries prefer to use stainless steel vessels.
As the yeast causes the natural sugar to ferment into alcohol, carbon dioxide gas bubbles off. Most breweries and larger distilleries can collect this by-product for industrial use. However, recycling the gas is not economically viable for smaller malt whisky distilleries.
After two to four days, the beer – or wash, as the distillers call it – contains around eight to nine per cent ABV (alcohol by volume). At this point, the fermented worth is ready to pass to the first distillation(-hyperlink). Finally, the residue produced at this stage, mash tun, often goes to local farmers to feed cattle.